Thermodynamic behavior of Na₂O-B₂O₃ melt

The activity of Na₂O in a Na₂O-B₂O₃ melt was measured at 1373 K by a chemical equilibration technique to understand the thermodynamic behavior of this slag system. Also, the activity coefficient of Na in Ag was preliminarily measured as fundamental thermodynamic data, to estimate the activity of Na₂O in the slag. Sodium in the silver melt, within the present concentration range, exhibits the Henrian behavior, and the Henrian activity coefficient of Na in Ag (γ_Na⁰) is estimated to be 37.5 at 1373 K, indicating a positive deviation from ideality. The activity of Na₂O in the slag varies from 1.86 × 10^-6 to 4.99 × 10^-4 when its content is increased from 11.0 to 64.9 mol pct; this indicates a significant negative deviation from ideality. The excess stability does not exhibit any pronounced peak, despite being drastically changed at specific slag compositions. Comparing the molar Gibbs free energy of mixing in various binary slags, Na₂O was considered to be more basic than BaO (CaO) was, followed next by MgO; also, P₂O₅ would be significantly more acidic than SiO₂ would be, followed next by B₂O₃. Comparing the heats of formation of solid compounds in binary slags, the larger the differences in the electronegativity values between slag components, the more the relative ionic characters of the melts seemed to be.